A wide variety of foam materials, or common "sponges", which effectively absorb moisture are well-known in commercial practice. Typically, such foams are open-cell structures and comprise various cellulosic or polymeric materials. For example, various polyurethanes and like materials have long been used to prepare synthetic foams. As is known in the art, foam materials function most efficiently as absorbents for aqueous liquids when their surfaces are substantially hydrophilic. However, many synthetic foams are prepared by the polymerization of organic monomers which yield polymeric foams which are substantially hydrophobic in nature. Accordingly, considerable attention has been given to finding means whereby otherwise hydrophobic synthetic foams can be rendered hydrophilic.
For example, it is known that some types of foams have been prepared using certain selected monomers which, themselves, impart at least some degree of hydrophilic character to the resulting polymerized foam. Such monomers are then incorporated into the basic structure of the foam network during the polymerization process. Unfortunately, the hydrophilic substituents present in the monomers can undesirably modify the basic characteristics of the resulting foam. Thus, while the resulting foam may have the desired hydrophilic character, it may lose some of its other desirable structural features or performance qualities. Moreover, such specialized, hydrophilic monomers can be expensive relative to standard monomers used to prepare foams, and thus their use can increase the overall cost of the foam.
In other processes, some foams have been treated to provide anionic substituent groups such as carboxylate or sulfonate moieties on their polymeric structures. Such anionic substituents can be effective in hydrophilizing the surface of the foams, but, unfortunately, their utilization can result in foams that are rather stiff and lack resilience. Such foams are not optimally comfortable when used in close contact with human skin, as, for example, in diapers and sanitary articles.
In some instances, synthetic hydrophobic foams can be rendered hydrophilic by incorporating small quantities of surfactants into the foam matrix. While this can render a foam hydrophilic and quite useful for some purposes, surfactant-containing foams are not always suitable for use in prolonged contact with skin, since the surfactant can cause skin irritation. In addition, some surfactants, e.g., water-soluble ones, can desorb from the foam and dissolve into the fluid being absorbed by the foam. This can significantly change the surface tension of the fluid and dramatically affect the strength with which it is held by the foam.
The manufacture of hydrophilic foams for use as fluid absorbents in sanitary articles, especially disposable diapers and sanitary napkins, requires that the foams not only have superior fluid-handling properties, but also be comfortable to the wearer and safe when used in close proximity to human skin over prolonged periods of wear. Moreover, it is important to the performance of foams designed for use in diapers and catamenials that the fluidity properties of body fluids such as urine and menses not be substantially affected by the hydrophilizing agent, such as could happen when some surfactants, e.g., water-soluble ones, are used to hydrophilize absorbent foams. Accordingly, safe, effective, economical means for hydrophilizing absorbent foams is of substantial interest to the manufacturer of such items. The present invention provides a safe and effective foam hydrophilization method which meets the foregoing requirements.